Optimize isNodeType

Check mixin types for direct match before talking to the
nodetypemanager. This avoids unnecessary calls to the backend.
Merge pull request #252 from jackalope/optimize_is_node_type

Optimize isNodeType
Ported the node validation logic from Doctrine-Dbal

Ported the transport agnositc node validation and processing logic
from Doctrine-Dbal to Jackrabbit.
Fixed NS validation
Updated exception messages.